To provide a better understanding of the technical aspects of the safety problem of boreholes communicating with underground reserves of fluid, reference is initially made to a relatively detailed example illustrated in the first four accompanying figures. These figures apply to a particular case of a storage facility using a cavity excavated by washing out rock salt. As shown in FIG. 1 which includes vertical graduations to give an idea of scale, cavities of this type may have volumes of up to about 5.times.10.sup.5 m.sup.3. A single borehole 2 connects the cavity 1 to the surface when operating units 3 are installed. To do this, the borehole often passes through salt 6, but also and mainly it passes through overlying sedimentary rock 5. The washing out technique for forming the cavity 1 consists essentially in injecting fresh water via a dip tube 10 and in recovering the water saturated with dissolved salt (known as brine) through the annular gap between the tube 10 and a temporary guard tube (not shown) installed in the borehole 2. This technique also gives rise to an accumulation of insoluble material 7 at the bottom of the cavity 1, and the cavity remains filled with brine.
Before being able to store gas therein, for example, the site operator must therefore remove the initial brine. This takes place in a stage prior to operation per se, and referred to in the art as "dewatering". This is shown diagrammatically in FIG. 2 in which fairly realistic relative dimensions are retained by means of two imaginary nearly horizontal section lines. This shows up more clearly the structure of the borehole 2 and the configuration of the tubes it contains specifically during this stage. Starting from the center, there is the dip tube 10 which extends down nearly to the bottom of the cavity, with its bottom end opening out immediately above the insoluble materials 7. It is referred to below as the "central" tube. It is surrounded by a casing system for the borehole 2 and delimiting an annular space 9. Still going towards the outside of the borehole, this system comprises: a protective tube 30 (also referred to below as a "peripheral" tube), metal casing 20, and a cement lining 25. The lining extends down to the shoe 21 at the bottom of the borehole 2 and ensures that the casing is firmly attached to the rock, and a brine-based liquid of appropriate density, often referred to as "completion liquid" in the art, is disposed between the protective tube 30 and the casing 20. This liquid is retained at the bottom by an annular packer 31 and serves to exert a supporting force on the casing 20 thus making it possible to reduce its crushing strength.
Dewatering then consists in injecting the gas G to be stored via the annular space 9. So long as its pressure is high enough, it pushes the gas/brine interface 8 downwards the bottom of the cavity, with the brine B thus being constrained to rise via the dip tube 10. In this way, the brine B within the cavity is replaced progressively by the gas G. When the cavity finally contains gas only, storage operation proper may begin. This generally lasts for 20 to 25 years, during which time gas is drawn off or replaced at widely spaced-apart intervals. For these operations it is usual for all or a part of the dip tube 10 extending inside the cavity to be removed. That is why this tube is referred to below as the central tube, i.e. with reference to its central position in the borehole. It is conventional for the gas that is taken away at the surface to be drawn off via this tube, and for the annular space 9 to be voluntarily closed off.
This method of operation can be explained by the need to provide a borehole in operation with safety devices that are suitable, in the event of an accident, for interrupting all communication between the cavity 1 under pressure and the surface. The normally-closed safety valves that are usually used for this purpose happen to be easier to fit to the central tube 10 and to be unsuitable for placing the annular space 9 without additional fittings. It is therefore common practice for operators to take the precaution of closing off this annular space as soon as dewatering has been accomplished. Various ways have been used in the past for doing this. One of them, commonly implemented at present, is shown in FIGS. 3 and 4 which are half-sections of the borehole. An annular mask 40 is attached to the inside of the central tube 10. The mask 40 has a telescopic portion 42 which is deployed to its maximum length (FIG. 3) or is fully retracted (FIG. 4) depending on whether oil under pressure is applied to a control line 44. When extended, the mask 40 isolates the inside of the central tube 10 from the annular space 9. Instead, the lateral orifices 11 and 12 formed through the central tube 10 are put into communication with each other so that the gas injected during dewatering, for example, can bypass a fixed connection provided between the central tube 10 and the protective tube 30. When the mask is retracted, one of the lateral orifices 11 is disengaged while the other lateral orifice 12 remains isolated by the mask. In these circumstances, the gas drawn off is diverted from the annular space 9 into the central tube 10.
Although this device does indeed contribute to providing safety for the storage facility while in operation by closing off the annular space 9, it nevertheless from several drawbacks. Firstly, prior systems inherently reduce the flow section available to gas being drawn off, since the total flow section of the central tube plus the annular space is restricted at the outlet to the section of the central tube only. In other words, above the safety valve, only a portion of the inside section of the borehole 2 is used, thereby increasing head losses in the corresponding gas flow and also increasing its flow velocity. In addition, these head losses are further increased by items comparable to the above-described mask 40 since they take up room inside the central tube and reduce its flow section. They also contribute to hindering insertion of downhole tools or withdrawal of tubes. Other ways of closing the annular space during the operating stage have also been designed that are not inserted in the central tube, and therefore do not restrict its section. However these other items suffer from other drawbacks such as the need to offset the central tube relative to the axis of the borehole, the need to provide compact safety valves so that they can be disposed directly in the annular space, the risk of the casing suddenly becoming detached under the effect of internal pressure, the risk of being impossible to remove, . . .
An object of the present invention is thus to optimize both dewatering and operation proper of the borehole, while nevertheless making it easy to use conventional means such as normally-closed valves of regular size for safety purposes during operation. This object is achieved without the above-listed drawbacks occurring.